What actually limits coax at QRO with high SWR

Most “SWR stress math” goes wrong because it treats SWR like a linear multiplier. In Walter Maxwell’s “Another Look at Reflections”, the key sentence is the square-root one: voltage at an SWR maximum is only √SWR times the matched value (under the conservative planning assumption).

What actually limits coax at QRO (regardless of the SWR number)

Coax fails from two real mechanisms:

• Peak voltage breakdown — arcing through dielectric or across connector/termination geometry.
• Heating — conductor loss and dielectric loss. Mismatch can create localized hot spots, so average power must be derated when SWR is high.

In practice, connectors, baluns/unun boxes, tuners, and contamination/moisture at terminations often fail before the coax itself does.

The conservative planning rule: √SWR

When you plan for worst case (same delivered power despite mismatch), the stress bound you should use is:

Matched reference formulas (50 Ω line)

• V_rms,matched = √(P · Z0)
• I_rms,matched = √(P / Z0)

1500 W example (clean √SWR math only)

Assume P = 1500 W and Z0 = 50 Ω.

Matched:

• V_rms,matched = √(1500 · 50) ≈ 274 V_rms
• V_peak,matched ≈ 274 · 1.414 ≈ 387 V_peak

SWR = 5: √5 ≈ 2.236

• V_rms,max ≈ 274 · 2.236 ≈ 612 V_rms
• V_peak,max ≈ 387 · 2.236 ≈ 866 V_peak

SWR = 20: √20 ≈ 4.472

• V_rms,max ≈ 274 · 4.472 ≈ 1225 V_rms
• V_peak,max ≈ 387 · 4.472 ≈ 1731 V_peak

Quick √SWR multipliers you can remember

What this means at HF legal limit

At HF, the matched line voltage for 1.5 kW into 50 Ω is only ~274 V RMS. With √SWR scaling, even “ugly” mismatch often lands in the low-kV range rather than the tens-of-kV fantasy numbers. So dielectric breakdown is usually not the first wall you hit (assuming clean/dry terminations, sane connector geometry, and proper weatherproofing outdoors).

Practical HF QRO guidance (√SWR lens)

• On decent HF coax (RG-8/RG-213 class), even 20:1 SWR at ~1.5 kW is generally not a coax dielectric breakdown problem by itself. The example above is ~1.7 kV peak under the conservative √SWR bound... typically still below common coax dielectric ratings, but terminations and hardware may be the weak link.
• Worry first about heating (loss + duty cycle) and weak points (connector cleanliness, moisture ingress, tuner/balun voltage capability).
• If something arcs, it’s often a termination/connector or a component box... not “the coax as a concept.”

Where people actually get hurt: heating + weak points

• Frequency: coax loss rises with frequency, so matched power rating drops fast as you go up in band.
• Duty cycle: SSB is forgiving; continuous modes are not.
• Hardware stress: connectors, tuners, baluns/unun boxes, and weatherproofing failures often decide the limit.
• Hot spots: mismatch can concentrate loss and temperature rise at specific points on the line.

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